Method and apparatus for vaporizing and superheating cryogenic fluid liquids

ABSTRACT

The present invention relates to methods and apparatus for economically vaporizing and superheating a stream of cryogenic fluid liquid. A body of heating medium liquid is heated in a closed vessel so that portions thereof are continuously vaporized. The stream of cryogenic fluid liquid to be vaporized and superheated is passed through a heating coil disposed within the vessel in heat exchange relationship with both the liquid and vapor portions of the heating medium so that the cryogenic fluid is vaporized and superheated to a desired level and so that vaporized heating medium is continuously condensed and returned to the liquid portion thereof.

United States Patent Arenson METHOD AND APPARATUS FOR Jan. 23, 1973 10/1968 Burrus et al. ..165/39 FOREIGN PATENTS OR APPLICATIONS [75] Inventor; E i Arenson, El Reno Okla 920,657 3/1963 Great Britain ..165/105 [73] Assignee: Black, Sivalls & Bryson, lnc., primary Examine, Meyer p i Oklahoma Okla Assistant Examiner-Ronald C. Capossela [22] Filed; F b, 3, 1971 Attorney-Jerry J. Dunlap, Wm. R. Laney, Robert M. 1 pp No: 112,174 Hessm and C. Clark Dougherty, Jr.

7] ABSTRACT ..62/52, l22/33ll76c5/7l/g; The present invention relates to methods and {581 Field of Search ..62/52, 53, 430; 165/105; f and g f 122/33 a stream of cryogenic flu1d liquid. A body of eating medium liquid is heated in a closed vessel so that por- [56] References Cited tions thereof are continuously vaporized. The stream of cryogemc fluid l1qu1d to be vaponzed and super- UNITED STATES PATENTS heated is passed through a heating coil disposed within the vessel in heat exchange relationship with both the 2,363,118 11/1944 Chamberlaln ..165/105 X d f h h 1,609,756 12/1926 MacMahon ..62/53X vapor, 0 t "5 2,055,949 9/1936 Sharp ..122/33 that cryogenc fluld and superheated 2,119,091 5/1938 Atkinson et al. ..122/33 {0 a desired level and 80 that vaporized heating medi- 2,539,29l 1/1951 Williamson et a1 ..62/52 um is continuously condensed and returned to the 2,618,935 11/1952 Ma1ir,.1r. ..62/52 X liquid portion thereof. 2,669,847 2/1954 Dick ..62/52 X 3,269,385 8/1966 Mitchell et al. ..62/52 9 Claims, 3 Drawing Figures OUT TAM/ 92 84 METHOD AND APPARATUS FOR VAPORIZING AND SUPERIIEA'IING CRYOGENIC FLUID LIQUIDS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to methods and apparatus for vaporizing cryogenic fluid liquids, and more particularly, but not by way of limitation, to methods and apparatus for vaporizing and superheating a stream of cryogenic fluid liquid by passing the stream in heat exchange relationship with a heating medium which is continuously vaporized and condensed within a single closed vessel.

2. Description of the Prior Art It is well known that it is economically advantageous to store and transport cryogenic fluids such as natural gas, oxygen, nitrogen and the like in the liquid state. Commonly, such fluids are refrigerated and liquefied at the site of their production and transported while in the liquid state to areas where they are to be utilized. The liquefied fluids are then revaporized and superheated to desired temperature levels at the areas of use. The term cryogenic fluid" is used herein to mean those fluids which exist in the liquid state at a temperature below about l50 F at pressures up to about 1,000 psia.

While various methods and apparatus have been developed for vaporizing cryogenic fluid liquids, many problems have been encountered due to the very low temperatures at which such fluids exist in the liquid state. For example, attempts have been made to vaporize cryogenic fluids such as liquefied natural gas utilizing conventional heaters in which the cryogenic fluid is conducted through tubes in the heater, and heat is transferred to the fluid by radiation and convection from combustion gases and flames therein. These attempts have been largely unsuccessful due to uneveness of heat transfer rate (flux rates) around the periphery of the heater tubes and the effect of the unevenness on the stability and uniformity of the boil.-

ing process within the tubes.

Because of the problems encountered with direct heating, methods of vaporizing cryogenic fluids indirectly have been developed. Commonly, thesemethods include directly heating a medium liquid such as isopentane in a heater, pumping or otherwise conducting the hot medium liquid to a heat exchanger and exchanging heat between the medium liquid and the cryogenic fluid to be vaporized. While these indirect heating methods are relatively successful in that stable operating conditions are achieved, they require elaborate apparatus to carry out which is expensive to manufacture, install and operate.

Heretofore, attempts have been made to vaporize and superheat cryogenic fluids directly with steam. That is, the cryogenic fluid is passed through heating tubes disposed in heat exchange relationship with steam only. While these attempts have achieved varying degrees of success, relatively costly apparatus is required due to the occurrence of a condition known in the art as vapor binding. When heat is transferred to a boiling liquid contained in heating tubes at too high a rate, a layer of vapor forms adjacent to and around the passing therethrough. This vapor binding condition results in a decrease of heat transfer to the liquid, and consequently, the boiling process within the tubes is unstable and uneven. Due to the very high heat transfer rate between condensing steam on the outside of heating tubes and boiling cryogenic liquid within the tubes, vapor binding readily occurs. Heretofore, because of vapor binding, it has been necessary to utilize a relatively high heating tube surface area in steam heating apparatus utilized for vaporizing and superheating cryogenic fluids making the apparatus relatively expensive. Further, stable operating conditions are difficult to achieve in such apparatus.

By the present invention, methods of vaporizing and superheating cryogenic liquids are provided which may be carried out using inexpensive apparatus of simple design and stable operation.

SUMMARY OF THE INVENTION The present invention relates to a method of vaporizing and superheating a stream of cryogenic fluid liquid comprising the steps of heating a confined body of heating medium liquid so that a portion thereof is continuously vaporized, passing the stream of cryogenic fluid liquid in heat exchange relationship with the heating medium liquid so that the cryogenic fluid is partially vaporized and then passing the cryogenic fluid in heat exchange relationship with the heating medium vapor so that cryogenic fluid is fully vaporized and superheated and so that the heating medium vapor is continuously condensed and returned to the body of heating medium liquid. Additionally, apparatus for carrying out the method of the present invention is provided.

It is, therefore, a general object of the present invention to provide methods and apparatus for vaporizing and superheating cryogenic fluid liquids.

A further object of the present invention is the provision of a method of vaporizing and superheating a stream of cryogenic fluid liquid which may be carried out in apparatus which is inexpensive to install and operate.

Yet a further object of the present invention is the provision of methods and apparatus for vaporizing and superheating a stream of cryogenic fluid liquid wherein vapor binding is prevented from occurring'and stable operating conditions are achieved.

Other objects and advantages of the present invention will be evident from the following detailed description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates, in diagrammatic form, a system for carrying out the method of the present invention,

FIG. 2 is a side elevational view of the vaporizer apparatus of FIG. I partially in section, and

FIG. 3 is a view taken along line 3-3 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and particularly to FIG. 1, a vaporizer apparatus which may be used for carrying out the method of the present invention is illustrated and generally designated by the numeral 10.

A stream of cryogenic fluid liquid from a conventional cryogenic fluid liquid storage tank 12, or other source, is pumped by a conventional pump 14 through conduit 16 into the vaporizer apparatus 10. While within the vaporizer apparatus 10, the stream of cryogenic fluid liquid passes through a heating coil, which will be described further hereinbelow, wherein heat is transferred to the cryogenic fluid causing it to be heated, vaporized and superheated. The superheated cryogenic fluid stream is removed from the vaporizer apparatus through conduit 18 from where it is conducted to a point of use or distribution.

Referring now to FIGS. 2 and 3, the vaporizer apparatus 10 of the present invention is shown in crosssection. The apparatus 10 basically comprises a closed horizontal vessel 20 having a forward end 22 and a rearward end 24. The forward end 22 of the vessel 20 includes an annular flange portion 26 having a circular cover plate 28 bolted thereto by a plurality of bolts 30. The rearward end 24 of the vessel 20 includes a flange portion 32 and a circular cover plate 34 is bolted thereto by a plurality of bolts 36. Conventional gaskets 35 are disposed between the flange portions 26 and 32 and cover plates 28 and 34 of the vessel 20.

A conventional U-tube fire box 38 is disposed in the lower portion of the vessel 20. The ends of fire box 38 extend through and are seal welded to the cover plate 28. A conventional natural gas or fuel burner 40 is disposed within one end of the fire box 38 and a stack 42 is attached to the other end thereof. Fuel is supplied to the burner 40 through conduit 44. As will be understood any convenient heat source may be utilized in the apparatus 10 in lieu of the fuel burning apparatus described above. For example, hot exhaust gases from a boiler, gas turbine or the like may be passed through a heating coil disposed in the lower portion of the vessel 20 for providing heat thereto.

A heating coil 46 is disposed within the upper portion of the vessel 20. Inlet and outlet nozzles 48 and 50 extending through and seal welded to the cover plate 34 are connected to the heating coil 46. As will be understood, the heating coil 46 is comprised of a plurality of heating tubes 52 which are serially connected together by return bends 54. The tubes 52 are serially arranged within the upper portion of the vessel 20 in a plurality of banks, one above the other. Preferably, as shown in FIGS. 2 and 3, the inlet nozzle 48 is connected to a first bank of tubes 60 positioned above the fire box 38. A second bank 62 is positioned directly above and connected to the bank 60, and a third bank 64 is positioned directly above and connected to the second bank 62. The third bank 64 is connected to the outlet nozzle 50.

A conventional fill nozzle 70 and drain and vent valves 72 and 74 are provided attached to the vessel 20.

OPERATION As illustrated in FIGS. 2 and 3, a quantity of heating gauge glass (not shown) for indicating the level of liquid within the vessel 20. Thus, the level may be adjusted by viewing the gauge glass and adding liquid to the vessel 20 through the opening or extracting liquid from the vessel 20 through the drain valve 72.

Referring particularly to HO. 1, after the heating medium liquid has been placed within the vaporizer apparatus 10, fuel is supplied through the conduit 44 to the gas burner 40 and ignited. The products of combustion from the burner 40 travel through the fire box 38 and are vented to the atmosphere through stack 42. Heat from the combustion within the fire box 38 is transferred through the walls thereof into the body of heating medium liquid causing it to be heated and a portion thereof to be vaporized. The heating medium vapors travel upwardly into the uppermost portion of the vessel 20 around the top bank of heating tubes 64 disposed therein.

Vent valve 74 is opened to vent air contained within the vessel 20 to the atmosphere. Once all the air has been vented from the vessel 20, the valve 74 is closed thereby confining the heating medium liquid vapors within the vessel 20.

A stream of cryogenic fluid liquid is pumped from the storage tank 12 by the pump 14 through the conduit 16, through the inlet nozzle 48 of the vaporizer apparatus 10 and into the heating coil 46 thereof. As will be described further hereinbelow, heat is transferred from the heating medium liquid and vapor contained within the vessel 20 to the stream of cryogenic fluid liquid passing through the heating coil 46 thereby vaporizing and superheating it. The vaporized and superheated cryogenic fluid exits and heating coil 46 through the outlet nozzle 50. From the outlet nozzle 50 the vaporized and superheated cryogenic fluid passes through conduit 18 to a point of use or distribution.

A temperature controller 82 which may be any conventional pneumatic or electric temperature controller, senses the temperature of the vaporized and superheated cryogenic fluid passing through conduit 18. The temperature controller 82 is operably connected to a conventional fuel gas control valve 84 disposed in conduit 44. In a well understood and conventional manner, the temperature controller 82 opens and closes fuel gas valve 84 in predetermined relation to the temperature of the fluid passing through conduit 18 thereby increasing or decreasing the volume of fuel gas supplied to burner 40 which in turn increases or decreases the amount of heat transferred to the cryogenic fluid stream passing through the vaporizer apparatus 10. Temperature controller 82 is set so that the stream of vaporized cryogenic fluid passing through conduit 18 is maintained at the desired level of superheat.

As the cryogenic fluid liquid stream passes through the heating coil 46, heat is transferred from the body of heating medium liquid 80 through the walls of the heating tubes container in the tube banks 60 and 62 and into the cryogenic fluid thereby heating it and vaporizing at least a portion of the liquid. The heated and partially vaporized stream then passes into the third bank of heating tubes 64. Heat from the heating medium vapor surrounding the third bank of heating tubes 64 is transferred through the walls of the heating tubes into the cryogenic fluid passing therethrough so that remaining liquid is vaporized and the vapor is superheated to a desired level of superheat. The heat transferred from the heating medium vapors in the uppermost portion of the vessel causes the heating medium vapors to be condensed and pass by gravity back into the body of heating medium liquid 80. Thus, in operation, portions of the body of heating medium liquid 80 are continuously vaporized, condensed on the bank of heating tubes 64 and returned to the body of heating medium liquid 80.

As will be understood by those skilled in the art, when a heating medium material in either liquid or the vapor state contacts the outside surface of heating tubes containing cryogenic fluid liquid, a fllm of the material forms on the tubes. This film acts as a barrier to heat transfer, and the outside walls of the tubes may approach the temperature of the cold fluid contained within the tubes rather than the temperature of the heating medium material on the outside of the tubes. Thus, for example, when cryogenic liquids are heated in a conventional heater with combustion gases containing water vapor, a fllm forms on the outside walls of the tubes preventing the outside walls from being heated above the freezing point of ice, and as a result, ice forms on the tubes.

The filming characteristics of various heat transfer materials are known in the art and have been characterized or indexed under what is known as the heat transfer coefficient. The heat transfer coefficient varies inversely with the resistance to heat transfer for a particular material. That is, the higher the heat transfer coefficient of a particular material, the less resistance to heat transfer imposed by the material. For example, combustion gases have a heat transfer coefficient of approximately to 40 btu/hr-ft -F. at the temperatures and other conditions found within a heater used to vaporize cryogenic fluid liquids. Condensing water vapor or steam, on the other hand, exhibits a heat transfer coefficient of approximately 1,000 to 3,000 btu/hr-ft -F. Thus, condensing steam does not form an appreciable barrier on the outside walls of heating tubes containing cryogenic liquids, and the walls are heated to a level approaching the temperature of the steam thereby preventing the formation of ice. By the present invention, a heating medium having a condensing heat transfer coefficient above 500 btu/hr-ft F. isutilized so that a minimum resistance to heat transfer is imposed by the heating medium.

While the maximum rate of heat transfer would be achieved by passing the cryogenic fluid solely in heat exchange relationship with the heating medium vapors, it has been found that a portion of the heating coil 46 must be positioned below the surface of the body of heating medium liquid 80 in order to insure that vapor binding does not occur. As previously described, it is essential that the heat transferred to the cryogenic liquid being vaporized within the heating tubes does not exceed the rate at which vapor binding occurs. By the present invention, this is accomplished by placing the portion of the heating coil 46 wherein the cryogenic fluid is heated to itsboiling point and at least partially vaporized beneath the surface of the body of heating medium liquid 80, and exposing to the heating medium vapor only the portion of the coil 46 wherein the remaining portion of the cryogenic fluid liquid is vaporized and the stream is superheated. Since the heating medium liquid forms a film of relatively high resistance to heat transfer on the outside of the heating tubes submerged therein, the transfer of heat to the liquid within the tubes is sufflciently low that vapor binding does not occur. After all or part of the vaporization has been completed at the low heat transfer rate, the stream of cryogenic fluid is subjected to the maximum heat transfer rate which occurs in the vapor section.

As will be understood by those skilled in the art, the particular portion of the heating coil 46 which should be submerged in the body of heating medium liquid will depend on a variety of factors such as the particular cryogenic fluid being vaporized and superheated, the inlet and outlet temperature and pressure of the cryogenic fluid, etc. By the present invention, however, the coil 46 is positioned within the vessel 20 in banks, one above the other. Thus, the vessel 20 may be initially filled with heating medium liquid so that banks 60 and 62 are beneath the surface of the body of liquid 80, and the bank 64 is in the vapor space above the liquid. After initial start-up and operation of the unit, if vapor binding occurs, additional liquid may be introduced into the vessel 20 thereby raising the level of the body of heating medium liquid and covering a portion of the heating tubes 52 in the bank 64. This in turn reduces the rate of heat transfer to the cryogenic liquid as it is being vaporized thereby preventing vapor binding. lf desirable, the level of the body of heating medium 80 may be lowered to the point where vapor binding occurs and then raised slightly to achieve maximum heat transfer rates without vapor binding.

Prior to the present invention the use of water as a heating medium liquid for vaporizing and superheating cryogenic fluids has been avoided due to the danger of the water freezing if the source of heat fails. Since the cryogenic liquid being vaporized and superheated enters the vaporizer apparatus at a very low temperature, if heat is not added to the heating medium liquid, it very quickly approaches the temperature of the cryogenic liquid. If water is used as the heating medium, ice will be formed causing severe damage to the apparatus 10. In order to overcome this problem, provision is made for dumping the water from the vessel 20 if the temperature thereof falls below a predetermined point. For example, referring to FIG. 1, a conventional temperature controller may be installed on the vessel 20 so that the temperature of the heating medium liquid within the vessel 20 is continuously sensed. The temperature controller is operably connected to a fuel gas shut-down valve 92 disposed in the fuel conduit 44 and a water dump valve 94 disposed in a conduit 96 connected to the bottom of the vessel 20. The temperature controller 90 is set at a temperature just above the freezing point of the heating medium liquid so that if for any reason heat is not continuously added thereto, the vaporizer apparatus will automatically shut down, i.e., fuel gas control valve 92 will close, and the heating medium liquid will dump out of the vessel 20, i.e., control valve 94 will open thereby dumping the water out of the vessel 20 through conduit 96, and damage to the vaporizer apparatus 10 due to the formation of ice and the expansion thereof will be prevented.

In addition to controls for dumping the water from the vessel 20, the bolts 30 and 36 holding the cover plates 28 and 34 to the ends of the vessel may be designed so that they will shear at a predetermined stress. Thus, if the body of heating medium liquid 80 reaches the freezing point thereof and begins to expand against the cover plates 28 and 34, the bolts 30 and 36 will shear allowing the cover plates 28 and 34 to move outwardly preventing damage thereto.

The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those inherent therein. While presently preferred embodiments of the invention are given for the purpose of disclosure, numerous changes in the details of construction and arrangement of parts can be made which will readily suggest themselves to those skilled in the art and which are encompassed within the spirit of the invention and the scope of the appended claims. What is claimed is: l. A method of vaporizing and superheating a stream of cryogenic fluid liquid comprising the steps of:

heating a confined body of heating medium liquid so that a portion thereof is continuously vaporized;

passing said stream of cryogenic fluid liquid in heat exchange relationship with said heating medium liquid so that said stream of cryogenic liquid is at least partially vaporized; and

then passing the cryogenic fluid in heat exchange relationship with the heating medium vapor so that the cryogenic fluid is fully vaporized and superheated and so that the heating medium vapor is continuously condensed and returned to said body of heating medium liquid.

2. The method of claim 1 wherein the heating medium liquid is a material which when vaporized has a condensing heat transfer coefficient of at least 500 btu/hrft -F.

3. The method of claim 2 wherein the heating medium liquid is water.

4. A method of vaporizing and superheating a stream of cryogenic fluid liquid comprising the steps of:

continuously heating a body of heating medium liquid in the lower portion of a closed vessel thereby vaporizing a portion thereof;

passing said stream of cryogenic fluid liquid through a heating coil disposed in heat exchange relationship with said body of heating medium liquid so that the stream of cryogenic fluid liquid is heated and at least partially vaporized; and

passing the stream of heated and partially vaporized cryogenic fluid through a heating coil disposed in the upper portion of said vessel in heat exchange relationship with said vaporized heating medium so that the stream of cryogenic fluid is fully vaporized and superheated and so that the vaporized heating medium is continuously condensed and returned to the body of heating medium liquid.

. The method of claim 4 wherein said heating medium liquid is a material which when vaporized has a condensing heat transfer coefficient of at least 500 btu/hrft-F.

6. The method of claim 5 wherein the heating medium liquid is water.

7. Apparatus for vaporizing and superheating a stream of cryogenic fluid liquid comprising:

a closed vessel having a lower portion for containing a body of heating medium liquid and an upper portion for containing heating medium vapor;

means for heating said body of heating medium liquid and continuously vaporizing portions thereof disposed within the lower portion of said vessel;

a first cryogenic fluid heating coil disposed within said vessel beneath the surface of and in heat exchange relationship with said body of heating medium liquid so that cryogenic fluid passing therethrough is heated and at least partially vaporized; and

a second cryogenic fluid heating coil disposed within said vessel above the surface of said body of heating medium liquid and in heat exchange relationship with said heating medium vapors, said second cryogenic fluid heating coil being serially connected to said first cryogenic fluid heating coil so that the heated and partially vaporized cryogenic fluid passing therethrough is completely vaporized and superheated and heating medium vapors are continuously condensed and returned to said body of heating medium liquid.

8. Apparatus for vaporizing and superheating a stream of cryogenic fluid comprising:

a closed vessel having a lower portion for containing a body of water and an upper portion for containing steam;

means for heating said body of water and continuously converting portions thereof into steam disposed within the lower portion of said vessel;

21 first cryogenic fluid heating coil disposed within said vessel beneath the surface of and in heat exchange relationship with said body of water so that cryogenic fluid passing therethrough is heated and at least partially vaporized; and

a second cryogenic fluid heating coil disposed within said vessel above the surface of said body of water and in heat exchange relationship with said steam, said second cryogenic fluid heating coil being serially connected to said first cryogenic fluid heating coil so that heated and partially vaporized cryogenic fluid passing therethrough is completely vaporized and superheated and steam is continuously condensed and returned to said body of water.

9. The apparatus of claim 8 which is further characterized to include means for dumping said body of water from said closed vessel when the temperature of said body of water reaches a preselected minimum level thereby preventing said body of water from forming ice and damaging said vessel. 

2. The method of claim 1 wherein the heating medium liquid is a material which when vaporized has a condensing heat transfer coefficient of at least 500 btu/hr-ft2-*F.
 3. The method of claim 2 wherein the heating medium liquid is water.
 4. A method of vaporizing and superheating a stream of cryogenic fluid liquid comprising the steps of: continuously heating a body of heating medium liquid in the lower portion of a closed vessel thereby vaporizing a portion thereof; passing said stream of cryogenic fluid liquid through a heating coil disposed in heat exchange relationship with said body of heating medium liquid so that the stream of cryogenic fluid liquid is heated and at least partially vaporized; and passing the stream of heated and partially vaporized cryogenic fluid through a heating coil disposed in the upper portion of said vessel in heat exchange relationship with said vaporized heating medium so that the stream of crYogenic fluid is fully vaporized and superheated and so that the vaporized heating medium is continuously condensed and returned to the body of heating medium liquid.
 5. The method of claim 4 wherein said heating medium liquid is a material which when vaporized has a condensing heat transfer coefficient of at least 500 btu/hr-ft2-*F.
 6. The method of claim 5 wherein the heating medium liquid is water.
 7. Apparatus for vaporizing and superheating a stream of cryogenic fluid liquid comprising: a closed vessel having a lower portion for containing a body of heating medium liquid and an upper portion for containing heating medium vapor; means for heating said body of heating medium liquid and continuously vaporizing portions thereof disposed within the lower portion of said vessel; a first cryogenic fluid heating coil disposed within said vessel beneath the surface of and in heat exchange relationship with said body of heating medium liquid so that cryogenic fluid passing therethrough is heated and at least partially vaporized; and a second cryogenic fluid heating coil disposed within said vessel above the surface of said body of heating medium liquid and in heat exchange relationship with said heating medium vapors, said second cryogenic fluid heating coil being serially connected to said first cryogenic fluid heating coil so that the heated and partially vaporized cryogenic fluid passing therethrough is completely vaporized and superheated and heating medium vapors are continuously condensed and returned to said body of heating medium liquid.
 8. Apparatus for vaporizing and superheating a stream of cryogenic fluid comprising: a closed vessel having a lower portion for containing a body of water and an upper portion for containing steam; means for heating said body of water and continuously converting portions thereof into steam disposed within the lower portion of said vessel; a first cryogenic fluid heating coil disposed within said vessel beneath the surface of and in heat exchange relationship with said body of water so that cryogenic fluid passing therethrough is heated and at least partially vaporized; and a second cryogenic fluid heating coil disposed within said vessel above the surface of said body of water and in heat exchange relationship with said steam, said second cryogenic fluid heating coil being serially connected to said first cryogenic fluid heating coil so that heated and partially vaporized cryogenic fluid passing therethrough is completely vaporized and superheated and steam is continuously condensed and returned to said body of water.
 9. The apparatus of claim 8 which is further characterized to include means for dumping said body of water from said closed vessel when the temperature of said body of water reaches a preselected minimum level thereby preventing said body of water from forming ice and damaging said vessel. 